1. Field of the Invention
This invention relates to data processing systems. More particularly, this invention relates to data processing systems including a plurality of processors which operate in both a split mode and a locked mode.
2. Description of the Prior Art
It is known to provide data processing systems having a plurality of processors and which operate in both a split mode and a locked mode. As an example, Fault tolerant Platforms for Automotive Safety critical applications (Baleani, Ferrari, et al, 2003) discloses such a system which operates in both a split mode and a locked mode. Such systems give the advantage of redundancy and fault tolerance when operating in the locked mode and yet permit higher multiprocessor performance when operating in the split mode. When operating in the locked mode, the processors must track each other accurately on a cycle-by-cycle basis (although there may be some offset). In order to achieve this, the processors are normalised before entering the locked mode. This typically involves running a sequence of normalising program instructions to place state parameters accessible within the programming model into a predetermined state as well as performing a hardware reset to normalise state parameters which are not accessible in the software model.
In order to ensure complete normalisation between the processors, the debug mechanisms in each of the processors will also be normalised upon entry into the locked mode. This will overwrite any debug state present before the switch into the locked mode and makes it difficult to debug the switching process itself.